Aluminiding



United States Patent 3,489,537 ALUMIN'IDING Newell C. Cook, Schenectady,N.Y., assignor to General Electric Company, a corporation of New York NoDrawing. Filed Nov. 10, 1966, Ser. No. 593,273 Int. Cl. C23b 5/30, 5/22US. Cl. 24-194 12 Claims ABSTRACT OF THE DISCLOSURE Aluminide coatingsare formed on specified metal compositions by forming an electric cellcontaining said metal composition as the cathode joined through anexternal electrical circuit to an aluminum anode using a specified fusedsalt electrolyte maintained at a temperature of at least 600 C., butbelow melting point of said metal composition. This cell generateselectricity, but if desired an may be impressed on the circuit providingthe density of the cathode does not exceed amperes/dmF. The aluminumdifliuses into the base metal to form a coating on the substratecomposed of aluminum and the substrate metal. This process is useful inmaking such coatings on the substrate metals.

This invention relates to a method for metalliding a base metalcomposition. More particularly, this invention is concerned with aprocess for aluminiding a base metal composition in a fused salt bath.

Aluminiding has been a valuable commercial process for many years and isusually accomplished by the immersion of base metals in molten aluminum.This method of aluminiding has the disadvantages in that the amount anddegree of aluminiding cannot be accuratel controlled, and it isdiflicult to avoid having free aluminum on the surface and some metalsdissolved in the aluminum.

I have discovered a method whereby I can aluminide base metals withoutthe above disadvantages.

I have discovered that a uniform tough, adherent aluminide coating canbe formed on a specific group of metals employing low current densities,that is, current densities in the range of 005-10 amperes/drnf".

In accordance with the process of this invention, the aluminum metal isemployed as the anode and is immersed in a fused salt bath composedessentially of a member of the class consisting of the alkali metalfluorides with a member of the class consisting of calcium, strontiumand barium fluorides and containing from 0.01- 5 mole percent ofaluminum fluoride. The cathode employed is the base metal upon whichdeposit is to be made. I have found that such a combination is anelectric cell in which an electric current is generated when anelectrical connection, which is external to the fused bath, is madebetween the base metal cathode and the aluminum anode. Under suchconditions, the aluminum dissolves in the fused salt bath and aluminumions are discharged at the surface of the base metal cathode where theyform a deposit of aluminum which immediately diffuses into and reactswith the base metal to form an aluminide coating. In the specificationand claims I use the term aluminide to designate any solid solution oralloy of alu- 3,489,537 Patented Jan. 13, 1970 minum and the base metalregardless of whether the base metal does not form an intermetalliccompound with aluminum in definite stoichiometric proportions which canbe represented by a chemical formula.

The rate of dissolution and deposition of the aluminum is selfregulating in that the rate of deposition is equal to the rate ofdiffusion of the aluminum into the base metal cathode. The depositionrate can be decreased by inserting some resistance in the circuit. Afaster rate can be obtained by impressing a limited amount of voltageinto the circuit to supply additional direct current.

The alkali metal fluorides which can be used in accord ance With theprocess of this invention include the fluorides of lithium, sodium,potassium, rubidium and cesium. However, it is preferred to employ aneutectic mixture of sodium fluoride and lithium fluoride because somefree alkali metal is produced by a displacement reaction and potassium,rubidium and cesium are volatilized with the obvious disadvantages. Itis particularly preferred to employ lithium fluoride as the fused saltbath in which the aluminum fluoride is dissolved, because at thetemperatures at which the cell is operated, lithium metal is notvolatilized to any appreciable extent. Mixtures of the alkali metalfluorides with calcium fluoride, strontium fluoride and/or bariumfluoride can also be employed as a fused salt in the process of thisinvention.

The amount of aluminum fluoride in the fused salt bath can be from 0.01to 5 mole percent. Preferably the amount of aluminum fluoride ismaintained at about 0.1 to 0.5 mole percent.

The chemical composition of the fused salt bath is critical if goodaluminide coatings are to be obtained. The starting salt should be asanhydrous and as free of all impurities as is possible or should beeasily dried or purified by simply heating during the fusion step.Because oxygen interfers, the process must be carried out in thesubstantial absence of oxygen. Thus, for example, the process can becarried out in an inert gas atmosphere or in a vacuum. By the termsubstantial absence of oxygen, it is meant that neither atmosphericoxygen nor oxides of metals are present in the fused salt bath. The bestresults are obtained by starting with reagent grade salts and bycarrying out the process under vacuum or an inert gas atmosphere, forexample, in an atmosphere of forming gas (10% H N nitrogen, argon,helium, neon, krypton or xenon.

I have sometimes found that even commercially available reagent gradesalts must be purified further in orde to operate satisfactorily in myprocess. This purification can be readily done by utilizing scrap metalarticles as the cathodes and carrying out the initial aluminiding runswith or without an additional applied voltage, thereby plating out andremoving from the bath those metal impurities which interfere with theformation of high quality aluminide coatings.

The base metals which can be aluminided in accordance With the processof this invention included the metals having atomic numbers of from 23to 29, 41 to 47, and 73 to 79 inclusive. These metals are, for example,vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium,molybdenum, technetium, ruthenium, rhodium, palladium, silver, tantalum,tungsten, rhenium, osmium, iridium, platinum and gold. Alloys of thesemetals with each other or alloys containg these metals as the majorconstituent, that is, over 50 mole percent, alloyed with other metals'asa minor constituenhthat is, less than 50 mole percent, can also bealuminided in accordance with my process, providing the melting point ofthe resulting alloy is not lower than the temperature at which the fusedsalt bath is being operated. It is preferred that the alloy contain atleast 75 mole percent of the metal and even more preferred, that thealloy contain 90 mole percent of the metal with correspondingly less ofthe alloying constituent.

I have also found that when the metal to be aluminided is vanadium,niobium, tantalum, chromium, molybdenum or tungsten, it is advantageousto conduct the aluminiding process in the absence of carbon, becausecarbon forms a very stable metal carbide on the surface of such basemetals thereby rendering it difficult to further aluminide the basemetal and giving less firmly adhering deposits; I have found that carboncan be removed from the fused salt bath by operating it as a cellemploying as a cathode, the metals such as vanadium or niobium, untilthe carbide coating is no longer formed on the surface of the metal.

Inasmuch as titanium, zirconium and hafnium are more electropositivethan aluminum, it is not possible to aluminide these metals whileoperating the cell as a battery. I have found, however, that titanium,zirconium or hafnium can be aluminided if a negative potential isapplied to the titanium, zirconium or hafnium cathode. By the termelectropositive is meant that titanium, zirconium and hafnium morereadily give up their electrons and become positive ions than doesaluminum. Thus, for example, if titanium is put in contact with a fusedsalt bath containing aluminum ions, the titanium loses its electrons tothe aluminum and displaces the aluminum ions from the fused salt bath.

I have found that when a negative potential of at least 0.1 volt isapplied to the titanium, zirconium or hafnium cathode, that these metalscan be aluminided. Otherwise, the aluminiding is not readilycontrollable inasmuch as such cathodes, when placed in contact with thebath, immediately begin to displace the aluminum ions with resultantloss of the base metal from the cathode. I have discovered that the lossof the base metal of the cathode can be prevented by placing a negativepotential on the cathode before it is immersed in the fused salt bath tocomplete the electrical circuit and start the aluminiding, immediatelyupon contact of the cathode sample with the fused salt.

I have also found that the displacement reaction will still take placeafter the titanium, zirconium or hafnium cathode has been aluminided, ifthe circuit is broken prior to removing the cathode from the bath. It istherefore essential if a controlled aluminiding is to take place, that anegative potential be maintained on the cathode at all times when thecathode is in contact with the fused salt electrolyte.

The amount of the negative potential necessary to be applied to thetitanium, zirconium or hafnium must be at least 0.1 volt and can be from0.1 to 5.0 volts, and is preferably from 0.5 to 2.0. After the cathodehas been inserted in the bath, it is, of course, necessary to see thatthe current density is within the above-defined limits if good aluminidecoatings are to be obtained.

The aluminiding of titanium, hafnium or zirconium must, of course, beconducted in the substantial absence of oxygen and carbon as set forthabove.

Inasmuch as aluminum has a melting point of 660 C. and the cell isoperated at temperatures above 660 C., it is necessary to employ as theanode an alloy of aluminum and nickel if a rod-type electrode is to beused. I have also found that aluminum can be employed in liquid form ifit is placed in a graphite basket which has been shielded by means of atightly woven monel screen to prevent carbon from contaminating thefused salt bath. I have also found that a nickel strip which has beenpreviously aluminided in accordance with the process of this inventioncan be employedas the anode.

In order to produce a reasonably fast plating rate and to insure thediffusion of the metal into the base metal to form an aluminide, I havefound it desirable to operate my process at a temperature no lower thanabout 600 C. It is usually preferred to operate at temperatures of from9001200 C. and even more preferably, from 900- 1100 C.

The temperature at which the process of this invention is conducted isdependent to some extent upon the particular fused salt bath employed.Thus, for example, when temperatures as low as 700 C. are desired, aneutectic of potassium and lithium fluoride can be employed. Inasmuch asthe preferred operating range is from 900 C. to 1100" C., I prefer toemploy lithium fluoride as the fused salt.

When an electrical circuit is formed external to the fused salt bath byjoining the aluminum anode to the base metal cathode by means of aconductor, an electric current will flow through the circuit without anyapplied electromotive force. The anode acts by dissolving in the fusedsalt bath to produce electrons and aluminum ions. The electrons flowthrough the external circuit formed by the conductor and the metal ionsmigrate through the fused salt bath to the base metal cathode to bemetallized, where the electrons discharge the aluminum ions as thealuminide coating. The amount of current can be measured with an ammeterwhich enables one to readily calculate the amount of metal beingdeposited on the base metal cathode and being converted to the metallidelayer. Knowing the area of the article being plated, it is possible tocalculate the thickness of the metallide coating formed, therebypermitting accurate control of the process to obtain any desiredthickness of the metallide layer.

Although the process operates very satisfactorily without impressing anyadditional electromative force on the electrical circuit, 1 have foundit possible to apply a small voltage when it is desired to obtainconstant current densities during the reaction, and to increase thedeposition rate of the aluminum being deposited without exceeding thediffusion rate of the aluminum into the base metal cathode. Theadditional E.M.F. should not exceed 1.0 volt and preferably should fallbetween 0.1 and 0.5 volt.

When it is desirable to apply additional voltage to the circuit in orderto shorten the time of operation, the total current density should notexceed 10 amperes/dmf At current densities above 10 amperes/dm. thealuminum deposition rate exceeds the diffusion rate and the base metalcathode becomes coated with a plate of pure aluminum.

Since the diffusion rate of aluminum into the cathode articles variesfrom one material to another, with temperature, and with the thicknessof the coating being formed, there is always a variation in the upperlimits of the current densities that may be employed. Therefore, thedeposition rate of the iding agent must always be adjusted so as not toexceed the diffusion rate of the iding agent into the substrate materialif high efiiciency and high quality diffusion coatings are to beobtained. The maximum current density for good aluminiding is 10 amperesper dm. when operating within the preferred temperature ranges of thisdisclosure. Higher current densities can sometimes be used to formcoatings with aluminum but in addition to the formation of a metallidecoating, plating of the iding agent occurs over the diffusion layer.

Very loW current densities (0.0l0.l amp/dm. are often employed whendiffusion rates are correspondingly low, and when very dilute surfacesolutions or very thin coatings are desired. Often the composition ofthe diffusion coating can be changed by varying the current density,producing under one condition a composition suitable for one applicationand under another condition a composition suitable for anotherapplication. Generally, however, current densities to form good qualityaluminide coatings fall between 0.5 and 5 amperes/dm. for the preferredtemperature ranges of this disclosure.

If an applied E.M.F. is used, the source, for example, a battery orother source of direct current, should be connected in series with theexternal circuit so that the negative terminal is connected to theexternal circuit, terminating at the metal being metallided and thepositive terminal is connected to the external circuit terminating atthe metal anode. In this way, the voltages of both sources arealgebraically additive.

As will be readily apparent to those skilled in the art, measuringinstruments such as voltmeters, ammeters, resistances, timers, etc., maybe included in the external circuit to aid in the control of theprocess.

Because the tough adherent corrosion resistant properties of thealuminide coatings are uniform over the entire treated area, thealuminum coated metal compositions prepared by my process have a widevariety of uses. They can be used to fabricate vessels for chemicalreactions, to make gears, bearings and other articles requiring hard,Wear and corrosion resistant surfaces, and other articles where closetolerances are needed. Other uses will be readily apparent to thoseskilled in the art as well as other modifications and variations of thepresent invention in light of the above teachings.

The following examples serve to further illustrate my invention. Allparts are by weight unless otherwise stated.

EXAMPLE 1 Lithium fluoride (9988 grams) was charged into a Monel liner(6" diameter x 17% deep x 5 thick) and fitted into a mild steel pot (6%diameter x 18" deep x A" thick) with top flange (11" diameter x thick).A cover plate (11" diameter x 1 thick) with a water cooling channel andholes for two electrode ports, a bubbler and a thermocouple well, wasthen attached with a gasket to the flange of the cell. The cell wasevacuated to 0.1 mm. pressure and the lithium fluoride melted (M.P.

ing conducted at 1000 C. in accordance with the details given in thefollowing table.

TABLE II Volts, anode polarity Current on.

n-n- H Current ofl.

Current on. Current ofi.

Very little salt adhered to the steel sample on removal and was easilyscraped off. The sample gained 0.059 gram which was exactly theoretical.The coating was shiny, smooth and had a thickness of 0.3 mil. Thecoating was much harder than the initial steel surface and resisted theaction of nitric acid much better than the nncoated steel. X-rayemissions spectra showed the surface to be high in both aluminum andiron and to be substantially free of any other metal.

EXAMPLE 3 Employing the procedure described in Example 2, afterinserting the aluminum-filled graphite crucible, a nickel cathode (6" x1" x 0.020") was aluminided at 1000 C. in accordance with the conditionsset forth in the following table.

846 C.). Argon was allowed to flow into the cell to break 40 TABLE IIIthe vacuum and then with a flow of argon to prevent air Volts, anodefrom diffusing into the cell, high purity aluminum fluoride Tune (mm')pmamy Amps (150 grams) was added to the lithium fluoride. A carbon 9.2550 anode diameter) completely surrounded by a Monel Z3828 {3 Currentcloth screen (from which it was electrically insulated) was 3 1.0 thenintroduced into the salt to a depth of 6 and the cell 1 8: 8&8 '3Current was run employing the cathodes shown in Table I to 0 removeoxygen and other impurities from the fluoride salt. 035 0 TABLE I Volts,Current Percent anode Density, Weight coulombic Run Time Temp., C.polarity amps/dm B gained efficiency (1) (Ni) 30min 980 +1. 7-2.2 1.20.060 36 (2) (Ni) min 1,000 +1. 6-2.3 1.2 0.200 60 (3) (Ni) 14 hrs 1,000+1. 2-1.6 0.2 1. 54 60 (4) (Fe) 28 hrs 1- 1,000 +2. 3-2. 0.4 1.5 16

Coulombic efliciencies are based on the cathode reaction Al+ +3e AlEXAMPLE 2 The sample was shiny and had almost no salt adhering to itwhen lifted from the fused salt bath. It had gained 0.030 gram ascompared to a theoretical gain of 0.028 gram. The coating showedimproved resistance to the action of nitric acid as compared to theaction of nitric acid on pure nickel.

EXAMPLE 4 Following the procedure described in Example 2, the followingmetals are aluminided at 1000 C. in accordance with the conditions setforth in the following table.

TABLE IV Current Weight Percent density gain, coulombic Run No. MetalTime amps/dm grams eflicicncy Description of clothing 1 C.R. Steel(1015) 15 hrs 0.2 0.896 100 6 mil. coat, light grey, smooth, hard,flexible, resistant to HNO and oxidation at high temperature. 2 Nickel0. 6 4. 486 100 2 mil. coat, grey, smooth, hard, brittle, very resistantto HNO 3 Cobalt... 5hrs 0.3 0.127 85 1 mil. coat, shiny, smooth, hard,brittle, very resistant to EN 4 Vanadium 104 min 0. 4 0. 035 100 0.5mil. coat, shiny, smooth, fair flexibility, very hard, very resistant toHNOQ, improved high temperature, oxidation resistance.

5 Chromium 117 min 1. 2 0.036 100 1 mil. coat, dark grey, smooth,brittle.

6 Molybdenum 12 hrs 0. 18 0.317 80 1.5 mil. coat, light grey, smooth,hard, very flexible, resistant to NHOa and elevated temperatureoxidation.

7 Niobium 20 min 0.3 0. 103 100 0.3 mil. coat, greyish brown, smooth,fairly hard, very flexible, marked improvement in air oxidationresistance.

8 Platinum min 5 0.028 100 0.5 mil. coat, shiny, smooth very flexible,hard.

9 Palladium 10 min 5 0. 026 92 0.3 mil. coat, shiny, smooth, veryflexible, hard.

10 Copper 63 min- 1. 0 0.378 100 3 mil. coat, shiny, golden colored,smooth, very flexible, slightly harder than base copper.

11 Cl(iro r7n)ium (18%) Iron 390 min 0. 4 0. 415 99 2.5 mil. coat,shiny, smooth, hard, flexible.

12 Kovar 110 min 1. 2 0. 113 98 1.2 mil. coat, shiny, smooth, very hard,flexible.

13 Monel 12 hrs 0.25 0.700 100 4 mil. coat, shiny, smooth, hard,flexible, very resistant to HN O 14 Stainless Steel (304) 230 min 0. 30. 310 100 3 mil. coat, shiny, smooth, hard, very flexible,

improved resistance to HNO 15 Titanlded Nickel. 60 min 1.0 0. 163 97 1mil. outer coatlng of Al, Ti and Ni; outer coat hardest and mostoxidation resistant.

16 Silicided Molybdenum 53 min 0. 0. 009 74 0.5 mil. outer coat ofAl,Si, and M0, 0.5 mil. inner layer of Si and Mo; both coatings hard andoxidation resistant.

17 4140 Steel 153 min 0.2 5.02 100 15flmill31coat, shiny, smooth,moderately hard,

exi e.

*The nickel sample was titanided in accordance-with the processdisclosed in application Ser. No. 593,275, filed concurrently herewithand assigned to the same assignee as the present invention and themolybdenum sample was siliclded by the process disclosed in the UnitedStates Reissue Patent 25,630, which references are made a part of thisapplication by reference. EXAMPLE 5 The cell described in Example 2 wasemployed in this example. Inasmuch as titanium, zirconium, and hafniumare all more electropositive than aluminum (i.e., Ti Ti+ +3e Al A1+-l-3e), it is necessary to keep a negative electrical force on thecathode at all times in order to prevent the displacement of aluminumions from the fused salt bath by the titanium, zirconium or hafniumcathode. This is accomplished by connecting a battery or other source ofdirect current in series with the metal electrodes of the cell beforethe titanium electrode is allowed to come into contact with the salt. Apotential of at least 0.1 volt is necessary to stop the displacementreaction, but a potential of 0.5 volt works better, and it can beconsiderably higher, i.e., up to 5 volts.

In operating this cell the negative potential was applied to the cathodeprior to immersing the cathode in the fused salt electrolyte to completethe electrical circuit. Of course, prior to completion of the circuit,no current was flowing. At the end of each run, the electrical circuitwas broken by withdrawing the cathode from the electrolyte prior todisconnecting the source of negative potential.

Table V summarizes the operating conditions and results of several runswherein titanium, zirconium and hafnium were aluminided.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A method of forming an aluminide coating on a metal compositionhaving a melting point greater than 900 C., at least mole percent ofsaid metal composition being at least one of the metals selected fromthe class consisting of metals Whose atomic numbers are 23 to 29, 41 to47 and 73 to 79, said 'method comprising (1) forming an electric cellcontaining said metal composition as the cathode, joined through anexternal electrical circuit to an aluminum anode and a fused saltelectrolyte which consists essentially of a member of the classconsisting of lithium fluoride, sodium fluoride, mixtures thereof andmixtures of said fluorides with calcium fluoride, strontium fluoride orbarium fluoride, said electrolyte being maintained at a temperature ofat least 900 C., but below the melting point of said metal compositionin the substantial absence of oxygen, (2) controlling the currentfiowing in said electric cell so that the current density of the cathodedoes not exceed 10 ampcres/dm. during the formation of the aluminidingcoating, and (3) interrupting the flow of electrical current after thedesired thickness of the aluminiding coating is formed on the metalobject.

2. The method of claim 1 wherein the fused salt elec- TABLE V Volts,Current Weight Percent cathode Time density, gain, coluombic Run No.Metal polarity (min.) amps/dm. grams eificiency Description of coating1.0 3 3 0. 050 96 0.5 coat, grey, smooth, flexible and springy. -5.0 510 0 270 96 1 mil. coating, grey, smooth, flexible and springy. 20..Betai'fgaiiur, 131% 2. 0 8 6 0. 035 80 0. 3 mil. coatlng, shiny, smoothflexible.

r, 21 z1roooi1im -2.0 10 rs 0. 099 so 0.5 mil. coat, grey, smooth, hard,flexible, marked improvement to air oxidation. 22 Hafnium 2.0 3 7 0.02080 0.5 mil. coat, shiny, smooth, flexible,

very hard, very oxidation resistant at high temperatures.

Before immersing cathode sample.

It will, of course, be apparent to those skilled in the art thatmodifications other than those set forth in the above examples can beemployed in the process of this invention without departing from thescope thereof.

trolyte consists essentially of lithium fluoride and aluminum fluoride.

3. The method of claim 1 which is also conducted in the substantialabsence of carbonaceous materials.

4. The method of claim 1 wherein the metal composition is nickel.

5. The method of claim 1 wherein the metal composition is cobalt.

6. The method of claim 1 wherein the metal composition is vanadium.

' 7. The method of claim 1 wherein the metal composition is molybdenum.

8. The method of claim 1 wherein the metal composition is niobium.

9. The method of claim 1 wherein the metal composition is iron.

10. The method of claim position is stainless steel.

11. A method of forming an aluminide coating on a metal compositionselected from the class consisting of titanium, zirconium or hafnium andalloys thereof wherein at least 50 mole percent of said alloy istitanium, zirconium or hafnium, said method comprising (1) forming anelectric cell containing said metal composition as the cathode, joinedthrough an external electrical circuit to an aluminum anode and a fusedsalt electrolyte which consists essentially of a member of the classconsisting of lithium fluoride, sodium fluoride, mixtures thereof, andmixtures of said fluorides with calcium fluoride, strontium fluoride orbarium fluoride and from 0.01-5 mole percent of aluminum fluoride, saidelectrolyte being maintained at temperature of at least 900 C., butbelow the melting point of said metal composition in the substantialabsence of oxygen, said cathode having a negative poten- 1 wherein themetal com- References Cited UNITED STATES PATENTS Re. 25,630 8 /1964Cook 20439 3,024,175 3/1962 Cook 20439 3,232,853 2/l966 Cook 204-393,024,176 3/1962 Cook 20439 2,828,251 3/1958 Sibert et a1. 204-39 OTHERREFERENCES I. Electrochemical Soc., v. 112, No. 3, p, 266.

ROBERT K. MIHALEK, Primary Examiner R. L. ANDREWS, Assistant ExaminerU.S. Cl. X.R. 204-39

